Go to MacGregorSailors.com
and there is a ton of info on this.
But, all you do is get it on the trailer, block it up so that it WILL NOT MOVE when you remove the keel bolt, the lower the blocking to remove the keel (letting out keel cable as needed and remove when appropriate). Many use floor jacks to build the V bracket on top of to hold the keel. Then you can wheel the keel to garage for epoxy/paint.
On the wallowing of the hole, picture a rectangle with a hole through it inches from the top long edge and about a foot from the short forward edge edge. With the keel up this hole wears toward the top long edge, vs the farther away short edge of the keel. When down it wears the long way, when up, it wears towards a dangerously thin piece of iron. So, now imagine that hole is no longer round, but oval. When up the keel falls to the top of the oval, now lower the keel, it is now lowering from a slightly different turning point and that causes the locking hole not to line up with the corresponding hole in the liner and keel box.
If it still isn't making sense, make a keel out of paper, put a hole for the locking hole and the keel hole. Put it on another paper and mark the holes with a pen. Now enlarge the keel hole to an oval, line it up with the old keel mark. Now you will see that when you pivot it, the locking mark will no longer line up.
Hope I am making sense. Go to the site and you can find many pics of people building rigs to remove keels and inspect. Also drawings with what I am describing.